This invention relates to an apparatus for monitoring patient respiration and heartbeat, and for processing representative signals received from the patient for the purpose of evaluating the condition of the patient's health; specifically, the invention provides an evaluation of the patient's respiration to detect the condition known as apnea, and to reliably detect this condition while rejecting false indications of the condition.
The medical term "apnea" means cessation of respiration or breathing. The apnea condition has become associated in recent years with the "sudden infant death syndrome", wherein a disturbing pattern of early deaths of apparently healthy infants has been noted. Studies and research programs have been conducted by agencies of the Federal Government and others in an attempt to identify the cause of sudden infant death syndrome, and such studies have revealed that infants who are subject to this syndrome are apparently not the healthy infants that they were once believed to be. These infants appear to have subtle anatomic and physiological defects of a neurologic, cardiorespirtory and/or metabolic nature. Evidence has developed that the syndrome is not caused by a single mechanism working at one moment in time, but rather by a number of developmental, environmental, and pathological factors which become combined in complex interactions and circumstances to set up a sequence of events that produces the sudden and unexpected infant death. Much of this research has revolved around the hypothesis that apnea during sleep is related to the syndrome. It has also been postulated that apneic episodes during sleep which do not necessarily lead to sudden infant death may lead to aberrations in central nervous system development. There is evidence that infants who have numerous apneic episodes during sleep will receive an inadequate oxygen supply to the brain, which may lead to retardation of brain development, and which in turn may lead to further loss of respirtory control and further apneic problems.
Apnea may be caused by a number of other factors not necessarily related to the condition of infants, some of which are spinal cord injury, muscular dystrophy, lung diseases, drug intoxication, and certain other risk factors which have become apparent in identifying those who might be candidates for apnea. In adults, a history of heavy snoring denotes an individual at risk to apnea, especially in combination with other factors such as obesity, underlying heart disease and/or high blood pressure.
Whereas apneic conditions may be monitored in hospital and laboratory environments by means of suitably connected electrodes to a patient's body, and monitoring of a cathode ray tube (CRT) display which exhibits a waveform related to patient respiration, such monitoring is impractical or impossible in less controlled environments. There is a need for techniques and devices which will enable appropriate monitoring of adults or infants and which will detect apnea in time to set off an alarm in order that the condition may be corrected. For sleeping patients, it is frequently only necessary to wake the patient by means of an audible alarm in order that the patient may become conscious and resume normal breathing. It is therefore important that techniques and devices be developed for utilization in conjunction with sleeping individuals, in order that apneic episodes may be detected and corrected before physiological damage or harm occurs. In all events, the detection of apnea requires a monitoring of respiration and/or heart rates. Such monitoring is accomplished by means of the attachment or coupling of suitable electrodes or other transducer devices to the patient, so that signals developed by such devices may be transmitted to circuitry for detection and analysis. Monitoring transducers are well-known in the art, usually taking the form of electrical contacts attached in good electrical conductivity with several points on the patient's body, together with a suitable electrical circuit for monitoring minute impedance changes and electrical voltages developed by respiration and cardiac activity. Other forms of transducers have been developed utilizing pressure sensing techniques, and which monitor pressure changes occurring in various parts of the body as body movement occurs during respiration. All of these transducers monitor a physical parameter which is correlated with respiration and/or heartbeat, and convert the parameter monitored into an equivalent electrical signal, and feed the signal into appropriate circuitry whose ultimate function is to note the occurrence of a respiration cycle and to perform such other functions as are deemed necessary.